Cactus ID Is Difficult

Hello!
I recently purchased a cactus to put on my windowsill to brighten up my room a bit, my problem is that I threw away the container right after I repotted it and now I don’t know what it is exactly. Could you help me?
The pot it’s in in this picture is four inches across and the ends of the spines are a dark red color.
Thank you so much!
Emma
Emma,
Hard to know for sure at that size, but I would guess a Gymnocalycium, although my 2nd guess would be a Ferocactus. If it blooms young, it’s probably a Gymnocalycium. Also, it looks like it could use more sun.
Peter